Gas turbines generally include a compressor, one or more combustors, a fuel injection system and a turbine. Typically, the compressor pressurizes inlet air which is then reverse-flowed to the combustors where it is used to provide air for the combustion process and also to cool the combustors. In a multi-combustor system, the combustors are located about the periphery of the gas turbine, and a transition duct connects the outlet end of each combustor with the inlet end of the turbine to deliver the hot products of combustion to the turbine.
Gas turbine combustors are being developed which employ lean premixed combustion to reduce emissions of gases such as NO.sub.X (nitrogen oxides). One such combustor comprises a plurality of burners attached to a single combustion chamber.
Each burner includes a flow tube with a centrally disposed fuel nozzle comprising a center hub which supports fuel injectors and swirl vanes. During operation, fuel is injected through the fuel injectors and mixes with the swirling air in the flow tube, and a flame is produced at the exit of the burner. The combustion flame is stabilized by a combination of bluffbody recirculation behind the center hub and swirl-induced recirculation. Because of the lean stoichiometry, lean premixed combustion achieves lower flame temperature and thus produces lower NO.sub.X emissions.
These premixed systems are susceptible to an unpredictable phenomena commonly referred to as "flashback." Flashbacks can be caused by any of a number of events, including ignition of impurities in fuel or ignition during mode switching when the flames are in a transient phase. When flashback occurs, a flame enters zones or cavities of the combustor chamber which may not be designed to contain flames. A flame can also move unexpectedly into combustor cavities used for firing modes other than the combustion mode being exercised at the time of the flashback occurrence. Both types of flashback occurrences result in a loss of combustion control and can additionally cause heating and melting of combustor parts, such as fuel nozzles, for example, that are not designed to withstand excessive heating. An operator generally has no method of recognizing the occurrence of a flashback until the combustor sustains damage.
Flashback is accompanied by a step change in emitted visible light from the flame in an area of the combustor where the flame should not exist. Some factors which can contribute to variability in the light profile include: fuel nozzle dimensions, combustion modes, location of sensor with respect to flame, and sensor integrity (aging effects, temperature effects, and fiber fouling).
Fiber optic sensors for combustion and industrial process monitoring and diagnosis in gas turbine and aircraft engine applications require rugged equipment and a high signal level. Generally such fiber optic sensors include large diameter sapphire or quartz rods or bundles of multiple fibers. These designs can be bulky, rigid, and expensive because of special components needed for coupling and packaging. For example, either a very long fiber bundle or a connector with special lenses is required to couple a fiber bundle sensing head to a remote electronic device, and these elements are lossy, bulky, and expensive. Similar coupling problems exist for sensors involving large diameter sapphire or quartz rods. Additionally, rods are too rigid to withstand mechanical and thermal stress for large mechanical systems which frequently undergo high temperature thermal cycles. During machining thermal cycles, dynamic vibrations, installation, and maintenance handling, large rods can crack.
Multiple optical fiber bundles are useful in some applications to provide a large light collecting area as well as redundancy in the event of fiber damage. Many packaging techniques, however, cannot withstand temperatures in excess of about 250.degree. C. Commercially available adhesives such as high temperature ultra-violet cured optical epoxies can withstand temperatures up to about 175.degree. C. Quartz tubing fused fiber bundles require heating the bundle to a temperature greater than 1500.degree. C. in order to melt the quartz. Silica fibers generally include germanium or fluorine dopants to provide desired numerical apertures. At above 700.degree. C., and particularly at above 900.degree. C., dopants in silica fiber cladding start to diffuse into the blank fused silica fiber core and the fibers then lose their original numerical apertures.
Therefore, 700.degree. C. is often used as the damage threshold for long term heating of silica fibers.
Detection circuitry must detect flashbacks and prevent false indications of flashbacks. A simple static comparator circuit (such as a limit switch) may have a limited lifetime as compared with the combustor and may require individual tuning of sensors and/or their data to cancel the effects of systematic variations on DC levels and AC levels such as mounting location, diode efficiency, and fiber optic cable/connector efficiency, for example.